This invention relates generally to an overload protector interposed between a voltage source and a load powered thereby, the protector functioning to effectively decouple the load from the source when the current drawn by the load exceeds a predetermined limit.
In many electrical circuit applications, the need exists for some form of current regulation to prevent excessive current flow from a power supply to a load. Current control is of particular importance in connection with electrical devices where a fault condition such as a shorted circuit may have destructive effects. Such destruction can take place in a matter of microseconds; hence fuses and electro-mechanical circuit-breakers have too slow a response time to afford the desired protection.
In one known form of rapid-response overload protector, an unregulated d-c input voltage from a primary source is applied to the load through a pass transistor in series with a current-limiting resistor to form a current-control network. The load may be any electrical device which draws current from the source, and the function of the protector is to cut off current flow to the load immediately when an abnormal condition arises, such as a shorted circuit.
In the protector, base current for the pass transistor is provided by a driver transistor connected to a latching transistor whose base is connected to the tap of a voltage divider shunted across the current-control network. Under normal load conditions, the driver transistor conducts to render the pass transistor conductive, whereas the latching transistor is biased to its non-conductive state because the voltage drop across the voltage divider is low. When, however, the current delivered to the load exceeds a predetermined limit indicative of an overload condition, the resultant voltage drop across the limiting resistor in series with the pass transistor increases to a point that biases the pass transistor to its "off" or non-conductive state.
As a consequence, the voltage drop across the divider becomes high and acts to apply a voltage to the latching transistor that renders it conductive. The resultant conduction of the latching transistor serves to bias the driver transistor to its "off" state, and this in turn renders the pass transistor non-conductive.
Thus when an overload current is drawn by the load, the resultant drop then produced across the limiting resistor cuts off the pass transistor, and this "off" condition is maintained by a latching action. In the latching state, current to the load is reduced to a very low value, this condition continuing as long as there exists a voltage drop between the voltage applied to the input of the protector and the voltage across the load or until primary power is removed.
The drawback of this form of protector arrangement is that an overload condition giving rise to a latching operation may be a transient or a condition of relatively brief duration which thereafter clears up. But because of the latching action, the protector continues to effectively decouple the supply from the load even though the load is then in an acceptable state.
In some circumstances, this drawback may have serious consequences; for when the electrical device which constitutes the load carries out a vital function, it is important that interruptions in the current supplied to the device be confined to those periods in which the device is not properly operative, as reflected by an overload condition, and that as soon as the device is properly operative, that it be fully powered to resume its normal operation.
The following patents, which relate to prior art forms of overload protectors, are of interest in connection with the present invention: U.S. Pat. Nos. 3,624,490; 3,597,657; 3,338,316; 3,026,469; 3,445,751 and 3,697,861.